This invention relates to controlling the light output of LED displays, and more particularly to controlling LED displays having drive current provided in the form of PWM pulses.
Where a light display is generated from the combined output of an array of red, green, and blue light emitting diodes (RGB LED array) the intensity of light output from the individual light emitting diodes must be closely monitored and controlled to achieve a desirable combined light output from the array. In many applications of such arrays, such as LCD monitors, it is preferred to drive the array with pulse width modulated (PWM) current pulses. By controlling the shape, duration, and frequency of the PWM pulses, the light output of the individual LEDs and the array can be closely controlled.
Prior control systems have utilized direct measurement of average light intensity, and in some cases have also attempted to utilize a measurement of forward drive current supplied to the LEDs, for controlling the light output of an RGB array. Difficulties in measuring the individual light outputs, and inaccuracies in current measurement caused by dealing with ripple current and rise and fall times of the current at the beginning and end of the PWM pulses have limited the effectiveness of such prior control systems.
Our invention provides improved control of an LED array by determining a constant relating the peak light output of an LED to the peak current of a PWM pulse driving the LED, and multiplying the average current of the PWM pulse by the constant to obtain a value for the average light output for the LED.
In one form of our invention, the constant is determined by simultaneously measuring peak light output of the LED and peak current of a PWM pulse driving the LED. The constant is then calculated by dividing the peak light output by the peak current of the PWM pulse. By making the simultaneous measurements at a time during the duration of the PWM pulse where the pulse has reached its full magnitude, rise and fall times of the pulse do not affect the measurements.
Determination of average current of the PWM pulse can be accomplished in a variety of ways. In one form of our invention, the average current of the PWM pulse is determined by integrating current in the PWM pulse over time. Determining average current in this manner further reduces the effect of rise and fall time on determining the average light output of the LED. Alternatively, the average current can be determined by sensing the current of the PWM pulse, and passing the sensor output through a low-pass filter, or an integrator, configured for producing an average current signal.
For arrays having two discrete colored LEDs driven by PWM pulses that partially overlap as a function of time, and having only a single sensor for measuring light output of the LEDs, our invention may be practiced by simultaneously measuring peak light output and current of one of the LEDs at a point in time when the PWM pulses do not overlap, simultaneously measuring the combined peak light output of both LEDs and the peak current of the PWM pulse driving the second LED at a time when the PWM pulses do overlap, and determining the peak light output of the second LED by subtracting the measurement of the light output of the first LED from the combined light output of both LEDs. The constants relating the peak light output to the peak current of each LED may then be calculated by dividing the peak light output of each LED by its respective peak current. The same methodology may be utilized in practicing our invention in arrays having more than two discrete colored LEDs.
The repetition rate for determining the average light output may be repeated as often as is required to obtain the accuracy desired for a given application. For applications having multiple LEDs, and single or multiple light sensors, our invention contemplates the use of multiplexing hardware or software for coordinating measurement and processing of the various measurements required for determining the constants and average currents. In some forms of our invention, the repetition rate for the measurements may be determined as a function of a measurable parameter, such as the temperature of the LED, or a heat sink attached to the LED.
We contemplate that our invention may be practiced as a method, or embodied in an apparatus, or embodied in a code on computer readable medium.
The foregoing and other features and advantages of my invention will become further apparent from the following detailed description of exemplary embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of my invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.